Mens rea: Latin for "guilty mind"; guilty knowledge or intention to commit a prohibited act.
Also: "a particular state of mind such as the intent to cause, or some foresight of, the results of the act or the state of affairs." (R v Daviault 1994 3 SCR 63 at para. 74)
Many serious crimes require the proof of mens rea before a person can be convicted.
In other words, the prosecution must prove not only that the accused committed the offence (actus reus) but that he (or she) did it knowing that it was prohibited; that their act (or omission) was done with an intent to commit the crime.
A maxim rich in tradition and well known to law students is actus non facit reum, nisi mens sit rea or "a person cannot be convicted and punished in a proceeding of a criminal nature unless it can be shown that he had a guilty mind".
Not all offences require proof of mens rea such as many statutory or regulatory offences.